Determining the spatial orientation, and/or its rate of change, for a body is useful in numerous different applications. One example is an airborne body such as an airplane or a helicopter, where the spatial orientation of the airborne body may be used in the navigation of the body from its current location to a desired location or in a desired direction. In operation, the spatial orientation information (including the orientation and its rate of change) may be provided to a person who may use the information to pilot the airborne body manually, or may be provided to a computer-controlled navigation system that controls the navigation of the airborne body. Another application in which the spatial orientation of a body is useful is in underground applications such as underground drilling for oil, natural gas and the like. In such underground applications, orientation information for an underground drill head that cannot be directly observed by sight, for example, may be useful or even necessary in navigating the drill in an intended direction or to an intended target location. It should be understood that these two general examples of applications in which spatial orientation information may useful are intended to be illustrative, and that there are many other present and future applications in which spatial orientation information may similarly be useful.
In the context of an airborne body such as an aircraft, for example, traditional techniques for determining spatial orientation information include on-board mechanisms such as a compass to determine a directional heading for the aircraft, and in addition inertial navigation equipment (accelerometers and gyroscopes, for example) that are capable of determining the aircraft's attitude, including the conventionally defined pitch, yaw and roll components of attitude. By way of background, pitch angle describes an upward or downward rotation of the nose of the aircraft, the roll angle describes a rotation about the body of the aircraft (or in other words, tilting upward or downward of the wings), and the yaw angle describes the angle of side rotation (or in other words, the nose of the aircraft moving to the right or the left). On-board navigation systems such as these traditional types may also provide information as to the rate of change in the aircraft's attitude. While in many applications, such on-board navigational systems are entirely sufficient, in many applications they may not be sufficiently accurate or responsive, by themselves. For example, one issue that may be present with inertial navigation systems is possible drift in its spatial orientation determinations over time, and as such there may be a need to periodically calibrate or correct for such drift. In addition, the rate of change in the spatial orientation may be so dramatic or rapid for a particular body that such spatial orientation determination mechanisms are simply ineffective by themselves.
Another more recently developed technique for determining spatial orientation information for a body uses electromagnetic transmissions received from an external transmission system, such as microwave cellular tower transmissions or satellite transmissions. One example class of such electromagnetic transmission systems is a global navigation satellite system (GNSS). A GNSS, such as the Global Positioning System (GPS) of the United States or the Global Navigation Satellite System (GLONASS) of Russia, consists of several orbiting satellites that each make electromagnetic transmissions that are received by a body, and are used by the body for navigation.
A common and primary use that is made of received GNSS transmissions is to determine the current position of the body. This is known quite ubiquitously, as being used by aircraft and automobiles for example. Generally in the GNSS's operation, a body receives transmissions from at least four different satellites of the GNSS, and uses those transmissions to determine the current position of the body. One method for determining the three-dimensional position (longitude, latitude, and altitude) of a body includes receiving electromagnetic signals transmitted by satellites or beacons positioned at known locations. Small electric receivers can be used to calculate position based upon these received electromagnetic signals. The transmissions of GNSS satellites typically include time-stamp information as to when the transmission was sent by the satellite, so that the receiving body is able to use a time of receipt to calculate the body's distance from the particular satellite. In addition, the transmissions of GNSS satellites also conventionally include the current position of the orbiting GNSS satellite, or more specifically, ephemeris data that describes the orbital information for the satellite. As such, the body is provided with information regarding the current position of each of the satellites whose transmissions it has received, and uses that information, plus the distance each satellite is away from the body, to determine the position of the body.
In addition to being used for determining position, GNSS's have also been used more recently to determine spatial orientation information for a body. To do this, a body has been equipped with a receiving and directional antenna system that is capable of detecting a precise direction, defined by a vector, from which the electromagnetic transmission was received. In particular, the receiving system with the directional antenna system in such systems is configured to detect a vector direction for the received GNSS transmission that is defined by two angles, namely, an azimuth angle and an elevation angle. Knowing the precise receipt vector for each of the GNSS transmissions enables spatial orientation information to be determined in these bodies. In particular, given the precise vector directions, in addition to the time of transit for each transmission from satellite to the body and the known position of the satellite that sent the transmission, a processing system is able to calculate spatial orientation information for the body, including for example conventionally defined yaw, pitch and roll components. As a drawback, however, receiving and directional antenna systems that are capable of detecting both an azimuth angle and an elevation angle are generally complex and impose space requirements aboard bodies whose orientation is being determined, as compared for example to simpler receiving and antenna systems.
Certain antenna configurations can also be used as rotation sensors to make measurements that determine the orientation of the body without the aid of motion or additional sensors. Such systems use the GNSS signals arriving at antennas on the body to determine the direction toward the GNSS satellites relative to the body orientation. Geometry implies that the direction from the body toward a single satellite can be described by specifying the three angles formed between the satellite and the three body axes. The cosines of these angles are used to specify a unique direction cosine vector for each satellite expressed in the coordinate system for a given frame. Traditional systems that use GNSS for attitude estimation will take measurements of the direction cosine vectors made in the body frame and compare these measurements to satellite positions known in the navigation frame. Another embodiment of such system will describe the angle between a designated body x-axis and the satellite as an “angle-off-bore-sight”. The angle-off-bore-sight can also be described as an “elevation” angle for the satellite relative to the plane defined by the body y and z axes. Given that the elevation angle has been measured, then the satellite direction cosine vector can be determined by measuring an additional “azimuth” angle that specifies how the satellite direction is rotated around the x-axis. Traditional systems determine the body orientation relative to the navigation frame by measuring the vector directions (i.e., both the elevation and the azimuth angles) toward at least two satellites, and using these body measurements and the known direction of the satellites in the navigation frame to derive the actual rotation of the body frame relative to the navigation frame.